1. Field of the Invention
This invention relates to a heating apparatus suitable for use as a toner image fixing apparatus mounted in an image forming apparatus such as a copying machine, a laser beam printer or a facsimile apparatus.
More particularly, this invention relates to a toner image fixing apparatus of a type which heats and fixes an unfixed toner image corresponding to desired image information formed and borne on the surface of a recording material (such as paper, printing paper, a transferring material sheet, an OHT sheet, glossy paper or glossy film) by a direct method or a transferring method, as a permanently secured image on the surface of the recording material bearing the image thereon, by suitable image forming process means such as electrophotography, electrostatic recording or magnetic recording by the use of a toner comprising heat-soluble resin or the like, and an image forming apparatus such as a laser beam printer or a facsimile apparatus carrying the same fixing apparatus thereon.
Still more particularly, this invention relates to an on-demand fixing apparatus suitable for use in a color image forming apparatus and low in cost as well as short in rising time (so-called warm-up time).
2. Description of Related Art
In recent years, coloring in image forming apparatuses such as printers and copying machines have been advanced.
1) As a fixing apparatus used in such a color image forming apparatus, the fixing by a heat roller by a heat roller having an elastic layer on a fixing member is well known. An example of a fixing apparatus using a fixing roller having such an elastic layer is shown in FIG. 12 of the accompanying drawings.
In this fixing apparatus, design is made such that a recording material P bearing an unfixed toner image t thereon can pass through a contact nip portion (fixing nip) N between two heating rollers comprising a fixing roller 101 and a pressure roller 102 rotatively driven in the directions of arrows and adjusted to a predetermined fixing temperature.
The unfixed toner image t, when it passes through the nip portion N, is heated and pressurized by the fixing roller 101 and the pressure roller 102, and is fixed as a completed image (permanently adhered image) on the recording material P.
Each of the rollers 101 and 102 is provided with a halogen heater H centrally thereof, and they absorb radiation energy generated from the heaters H by aluminum mandrels 101a and 102a inside the respective rollers and are heated. Thermistors 103 and 104 are resiliently brought into contact with the surfaces of the respective rollers 101 and 102, and the electrical energization of the halogen heaters H of the respective rollers 101 and 102 is controlled on the basis of temperatures detected by the thermistors 103 and 104, and temperature adjustment is effected.
Elastic layers 101b and 102b of silicone rubber having a thickness of 2 mm are provided around the aluminum mandrels 101a and 102a, respectively, of the rollers 101 and 102, and on the outer surfaces of the rollers, there are further provided coating layers 101c and 102c of resin good in mold releasing ability and heat resisting property such as PFA (tetrafluoroethylene perfluoroalkyl ether copolymer/tetrafluoroethylene perfluoroalkyl vinylether copolymer resin), or FEP (tetrafluoroethylene hexafluoropropylene copolymer/ethylene tetrafluoride propylene hexafluoride copolymer resin) in order to prevent the toner, paper dust, etc. from being secured thereto.
It is for fixing the surface of the toner image as uniformly as possible that in the fixing nip portion N, the elastic layer 101b is provided on the fixing roller 101 side which is a fixing member contacted by the unfixed toner t.
By the elastic layer 101b being provided on the fixing roller 101 side, the elastic layer 101b is deformed along the toner layer when the toner image t passes through the fixing nip portion N, whereby the toner non-uniformly borne on the image is wrapped by the elastic layer 101b and is uniformly given heat, whereby uniform fixing is achieved.
The thus uniformly fixed image has the feature that it is free of uneven gloss, and particularly is excellent in the light transmissivity of the image when an overhead projector transparent (OHT) sheet is fixed.
However, the fixing apparatus of the heat roller type having such an elastic layer has suffered from the problem that the heat capacity of the heat roller itself becomes great and the time (warm-up time) necessary for the fixing roller 101 to be raised to a temperature suitable for toner image fixing is long. Also, the cost of the fixing member has been high.
2) On the other hand, as a fixing apparatus which is short in the warm-up time and inexpensive, there is well known a fixing apparatus of a film fixing type used in a black-and-white printer or the like. An example of such a film fixing apparatus is shown in FIG. 13 of the accompanying drawings.
This fixing apparatus is of a construction in which thin fixing film 111 is interposed between a heater 112 fixedly supported by a supporting member 115 and an elastic pressure roller 114 to thereby form a fixing nip portion N, the fixing film 111 is slidingly moved on the surface of the heater 112, a recording material P bearing a toner image t thereon is nipped and transported between the fixing film 111 and the pressure roller 114 in the fixing nip portion N, and the toner image on the recording material is heated by heat from the heater 112 transmitted through the fixing film 111. The unfixed toner image t on the recording material P receives heat and pressure when it passes through the fixing nip portion N, and is fixed as a completely fixed image (permanently adhered image) on the recording material P.
As the fixing film 111, use is made, for example, of endless film of heat-resistant resin having a thickness of the order of 50 μm, and a mold releasing layer (such as a fluorine resin coating layer) having a thickness of 10 μm is formed on the surface thereof, and the heater 112 comprises a ceramic substrate and a resistance heat generating member formed thereon. Temperature detecting means 113 is brought into contact with the heater 112, whereby the temperature of the heater 112 is detected, and temperature control is effected by controlling means, not shown, so that the temperature of the heater 112 may become a predetermined temperature.
Also, in order to make the heat capacity of the fixing film 111 small, an elastic layer is not provided on the fixing film 111.
In the fixing apparatus of such a construction, the heat capacity of the fixing film 111 is very small and therefore, it is possible to raise the temperature of the fixing nip portion N to a temperature capable of fixing the toner image within a short time after electric power has been supplied to the heater 112.
However, when the film fixing apparatus using such fixing film 111 provided with no elastic layer is used as the fixing apparatus of a color image forming apparatus, the surface of the fixing film 111 cannot follow the surface of the recording material P, the unevenness thereof due to the presence or absence of the toner layer, the unevenness of the toner itself, etc., and between a convex portion and a concave portion, a difference occurs to the heat applied from the fixing film. In the convex portion which is in good contact with the fixing film, the heat is well transmitted from the fixing film, and in the concave portion, as compared with the convex portion, it is difficult for the heat from the fixing film to be transmitted.
In a color image, toner layers of a plurality of colors are superimposed and mixed for use and therefore, the unevenness of the toner layers is great as compared with a black-and-white image, and when an elastic layer is absent on the fixing film which is a fixing member, the unevenness of the gloss of a fixed image becomes great to thereby deteriorate the quality of the image, and when the recording material is OHT, transmissivity has been bad when the fixed image has been projected, and this has caused the deterioration of the quality of the image.
So, there has been proposed a fixing apparatus using a fixing belt (fixing film) having an elastic layer in a film fixing apparatus to thereby constitute a low-cost color on-demand fixing apparatus (see, for example, Japanese Patent No. 3051085).
On the other hand, when use is made of the fixing method as described above, it is necessary that the fixing film or the fixing belt be driven to rotate or move and be transported while being slidden on the surface of a heater as a fixedly supported heating member and therefore, for the purpose of reducing the sliding friction with the surface of the heater, grease of the heat-resistant fluorine origin or the like is applied as a lubricant to between the two.
In this case, because of its characteristic, the grease is great in the grease viscosity during a low temperature and therefore, when image forming is executed in a case where for example, the image forming apparatus is left as it is for a predetermined or longer time under a low-temperature environment and the fixing apparatus is in a very cold state, the momentary torque at the starting of the fixing apparatus becomes great because the grease viscosity is in a great state, and the film or the fixing belt may sometimes slip.
Generally, in such a case, there is known a method of energizing the heater before the fixing apparatus is driven, and energizing a motor in a state in which the grease has been sufficiently melted to thereby prevent the slip, but when the heater is energized in a state in which the fixing apparatus has been warmed up earlier than the motor is energized, the temperature of the nip portion becomes higher than necessary, and this has caused hot offset or the damaging of the fixing apparatus in some cases.
So, there has been proposed a method of detecting the temperature of the heater by a temperature detecting element at the starting, and when it is judged on the basis of the result of the detection that the temperature is a predetermined temperature or less, effecting the judgment of the energization of the heater, and electrically energizing the heater before the rotation of the motor is started, and melting the grease to thereby reduce the starting torque.
However, the heat conductivity of silicone rubber or the like used for the elastic layer of the fixing belt is not very high and many members intervene between the surface of the fixing belt and the temperature detecting means for the heater and therefore, responsiveness is bad and it is difficult to effect the temperature control of the surface of the fixing belt by the temperature detecting means for the heater. Particularly, it is difficult to detect by the temperature detecting means for the heater that the transferring material has passed through the fixing apparatus and has taken away the heat of the surface of the fixing belt, whereby the temperature of the surface of the fixing belt has been lowered, or too much time is required of response.
Against such a problem, there has been proposed a method of displacing the disposition of the temperature detecting means from the heater portion to the surface or the inner surface or the like of the fixing belt, and detecting the temperature of the fixing belt itself to thereby control the driving of the heater and effect temperature control.
In this method, however, the temperature control of the fixing belt can be done accurately, but the temperature of the heater itself cannot be detected accurately, and there occurs a case where the temperature of the heater is too much lower or too much higher than the detected temperature by the temperature detecting element.
When as described above, the temperature detecting element is displaced to the surface or the inner surface of the fixing belt in order to accurately detect the temperature of the fixing belt and effect temperature control, the temperature of the heater cannot be directly detected and therefore, there arises the problem that the control of the timing of the energization of the heater and the energization of the motor becomes inaccurate. Thus, due to the unevenness of the heater temperature detection accuracy at the starting, there occurs a case where in spite of the heater itself being at a high temperature, the heater is energized before the motor is energized, and in such a case, the heater excessively rises in temperature and this has caused the occurrence of a faulty image such as hot offset or the deformation or damaging of the elastic layer by the excessive temperature rise, or has given rise to the problem that a member holding the heating member is melted or the heating member is damaged.